A. Do not idle the engine for excessively long periods. Long periods of idling
(more than 10 minutes) can damage an engine because combustion chamber temperatures
drop so low the fuel will not burn completely. This will cause carbon to clog
the injector spray holes and piston rings, and can cause the valves to stick.
If the engine coolant temperature becomes too low (60 degrees C [140 degrees
F]), raw fuel will wash the lubricating oil off the cylinder walls and dilute
the crankcase oil; therefore, all moving parts of the engine will not receive
the correct amount of lubrication. (For more information, refer to Cummins Operation
and Maintenance Manual section 1-5, Bulletin 3810205-12.)

From Josh Berman to the Cummins mail list:

The only time when extended idling becomes a problem
is when it is cool-to-cold outside. At that point, the engine may not be able
to generate enough heat to stay warm inside. When that happens, fuel can condense
on the inner walls of the cylinders, washing away the oil film and causing accelerated
wear. Carbon from the incompletely burned fuel can also clog injector tips and
stick the piston rings.

If it was extremely hot, you had the A/C running full
blast, and you were parked right up to a wall so there was no airflow through
the radiator, you might have an overheating problem (a bad thermostat notwithstanding),
but it's have to be pretty toasty outside for that to happen.----------------------------------------------------------------------
Joshua Berman MidRange
Service Cummins Engine Company
j.e.berman@metc.cummins.com
Cummins Homepage: www.cummins.com

"Cummins engines are designed to operate successfully at FULL THROTTLE
under transient conditions down to peak torque engine speed (edit by D Fritz:
The torque peak occurs at 1600 RPM for the B series engine used in the Ram;
the torque peak occurs at speeds as low as 1100 rpm for some larger engines).
This is consistent with recommended driving practices for good fuel economy.
Excessive FULL THROTTLE operation below peak torque RPM will shorten engine
life to overhaul, can cause serious engine damage, and is considered engine
abuse."

So you're right, we don't recommend FULL THROTTLE operation below peak torque
RPM. However, just after the first caution, the manual says:

"Operation of the engine below peak torque RPM can occur during gear
shifting due to the difference of ratios between transmission gears, but engine
operation must not be sustained more than 1 minute at FULL THROTTLE below peak
torque RPM."

Engine "lugging" is defined as operating under a load great enough that engine
speed can not increase at wide open throttle (assuming that the governor is
not cutting the throttle).

Generally, you can cruise around town in the Ram at 1100 rpm with no trouble,
but you should avoid using much throttle at such low engine speeds.

To reduce engine vibration that loosens the 5th gear nut:
Avoid running below 1600 rpm in 5th gear. With an enhanced 12V or any 24V engine,
keep the boost under 20 psi in 5th gear (OD) until the engine is turning above
1800 RPM.

Even with an unmodified engine, EGT can soar into the danger zone when the
engine is run at full throttle for several minutes. Always keep an eye on the
EGT when the engine has a heavy load and downshift when EGT reaches the top
of the safe range (950 at turbo elbow, 1250 at manifold).

These are conservative guidelines that help maximize engine and transmission
life. Dave

> We found ourselves pulling against some strong winds this past weekend> Buried in the carpet it'll do 65 in 3rd but it's right on the 2500
rpm governed limit> and I'm not sure how good it would be to run it there mile after
mile. Is running> with the throttle buried is a problem?

Running @ full throttle for several hours should not be a problem. We routinely
test the 6B at rated speed and power (ie: for a '96/'97 engine, the dyno pulls
215 HP out of the engine @ 2500 RPM) for several hundred hours at a stretch
(shutting down only to change oil or measurement equipment) in our test cells,
without problems.

> I have read in several posts that the v8 navistar engine that Ford> uses is a higher revving engine than the Cummins 5.9. What does the> 5.9 redline at? Since the 7.3 navistar redlines around 3300, The
5.9> should be much less per the posts.

The ISB should survive a _BRIEF_ excursion to 4000 RPM, and the
12-valve B to 3900 RPM. Do NOT do this on a regular basis - it is considered
abuse. However, if you miss a shift, or some other mistake, your engine probably
won't need major work if you don't go over those RPMs.

The valves begin to float at speeds a little higher than these, and that means
the valves hit the top of the pistons, usually leading to expensive repairs...

Regarding whether it's "better" or not to run an engine at a higher
or lower RPM, where better is defined as "engine not working as hard".

Before we get wound up, here's a simplistic assertion: diesel engine wear is
nearly totally a function of fuel consumption. But we need an accepted definition
of "working hard".

The effort required to maintain speed is the rate of power required at any
moment. Presuming that the situation remains constant (flat ground, no
traffic, etc) the rate of power required to maintain the speed will remain fixed
also. Observe in particular that this isn't a measurement of torque. Torque
can be more with fewer RPM, or less with more RPM, and achieve the same goal
of maintaining speed.

Now back to the fuel consumption. Sure, it's a generalization, and an
indirect one at that. What really matters of course is the amount of time spent
at high stress levels. Maximum stress levels are only achieved with the pedal
down, and generally, the further down the pedal, the higher the stress on the
engine, and the fewer the number of hours AND MILES the engine will operate.
Of course, if your engine will operate for 200k miles with the pedal always
to the metal, you might not give a rat's if it is being "consumed"
by the occasional stretch. For this discussion however, that's irrelevant.
We're trying to keep our engine from "working hard".

So from the above, accept that given two axle choices (3.54 vs 4.10) we're
trying to minimize fuel consumption.

One of the problems we face when looking for hard references on this question
is that most/all referenceable measurements are made at full throttle. That's
not what we care about. However, there is a clear trend: the best fuel
consumption, as measured by (fuel volume / hp), is at or near torque peak. I've
never seen any evidence that this does not hold at lower throttle settings.
So if we're trying to minimize fuel consumption to achieve a particular hp,
we'd be best to get the rpm down to near torque peak. Generally, this
tends to indicate a higher axle ratio (e.g. 3.54 rather than 4.10).

Now, there's lots more to the story than that of course. Again, if your engine
lasts twice as long as you own the rig, do you care that it doesn't last 3 times
as long?

Maybe more importantly, when operating at higher RPM at light throttle, there's
a lot more hp to be had by putting the pedal down than when operating at torque
peak. The rig is simply more responsive, and many people particularly those
who tow find this responsiveness to be an overwhelming part of their decision.